1. Field of the Invention
The present invention relates to a radio communication system including a plurality of portable or mobile units and a plurality of stationary base units which are connected to wire lines so as to be coupled through spatial radio circuits to the plurality of base units, and a method of controlling the system. More particularly, the present invention relates to a radio communication system in which each of a plurality of mobile units can know the busy state of an associated wire line, and a method of controlling the system.
2. Description of the Related Art
There has been so far demanded such a radio telephone system of public type that a plurality of stationary base units connected to wire lines are disposed at different locations and a plurality of portable or mobile units are provided in the service area of assocaited one of the plurality of base unit so that the plural mobile units can commonly use the wire line connected to the associated base unit. For the easy understanding of the public type radio telephone system, explanation will be made as to a prior art car telephone system which is considered to be similar to the public radio telephone system. The prior art car telephone system is configured as shown in FIG. 11, in which a public telephone network (PSTN) 101 is connected with a control station 102 which in turn is connected through a group of wire lanes 103a, 103b and 103c to base stations 104a, 104b and 104c respectively. The base stations 104a to 104c have respective service areas 105a to 105c which cover mobile stations 106a to 106e to allow these mobile stations to perform communication with the parties. For example, the mobile station 106a located within the service area 105a is connected through a spatial radio circuit, base station 104a, wire line 103a and then control station 102 to the public telephone network 101, whereby a communication line between the mobile station 105a and the party established. It is assumed in the system of FIG. 11 that the mobile station 106a moves at a high speed, the service areas 105a to 105c of the base stations 104a to 104c are set to have respectively a wide range corresponding to a circule area of a radius 10 km, and the system is arranged so that, when the mobile station 106a moves out of the service area 105a into the service area 105b, this causes switching of the system to its communication state based on the base station 104a from a communication state based on the base station 104b. Further, though the wire lines 103a to 103c have been illustrated only three in FIG. 11, an increased number of wire lines are provided so that the service areas 104a to 105c can cover many mobile stations, that is, so that the many mobile stations can be assigned to the associated wire lines. The car telephone system is essentially different in the following respects from the public type radio telephone system which is included in the present invention.
That is, the public type radio telephone system of the present invention requires three conditions (1) to (3) as follows.
(1) The movement distance of the user (mobile station) should be as short as possible. PA0 (2) For the purpose of realizing the use of a small battery as the power source of the mobile unit, the power consumption (in particular, power consumption for signal transmission) of the unit should be small. PA0 (3) In order to avoid radio interference between adjacent service areas, the signal transmission power of the mobile unit should be small.
For satisfying these conditions, it is desirable to reduce the service area, e.g., down to a circle area of radius 200 m. However, in the reduced service the area, possibility that many mobile units communicate with their parties at the same time is very small. Thus, it is uneconomical to assign a single base unit to a multiplicity of wire lines. For this reason, for example, one wire line is assigned to one base unit. In this case, while one mobile unit in the service area of the associated base unit is talking with the party, another mobile unit in the same service area cannot be used until the talk of the mobile unit in use is finished. This is similar to the situation where users are standing in front of a wired public telephone set in a queue awaiting their turn to use it. However, the public type radio telephone system of the present invention is different from the wired public telephone set in that the waiting users do not await their turn in a queue in front of the wired public telephone set but await the completion of the talk of the user in use at any location in the same service area. For this reason, the users have to await the completion of the talk of the user in question, without knowing how many other users are waiting and when the wire line becomes unoccupied.
In this way, the public type radio communication system has had such a problem that, in the event where a user wants to use his mobile unit but cannot use it due to the fact that the associated base unit is occupied by another mobile unit user, the user has to await the completion of the talk of another user without knowing the busy state of the base unit.